Plastic film for optical applications, polarizing plate, and image display device

ABSTRACT

Provided is an optical plastic film that can have good scratch resistance against both a hard material and a soft material. An optical plastic film has a first surface and a second surface located on the opposite side to the first surface, wherein the indentation hardness of the cross section on the first surface side, the indentation hardness of the cross section on the second surface side, and the indentation hardness of the cross section in the middle in the thickness direction satisfy a predetermined relationship in the conveyance direction and the transverse direction of the plastic film.

TECHNICAL FIELD

The present invention relates to an optical plastic film, a polarizingplate, and an image display device.

BACKGROUND ART

In most cases, various optical plastic films are used for opticalmembers of image display devices or the like. For image display devicesincluding a polarizing plate on a display element, plastic films(polarizer-protecting films) for protecting a polarizer constituting thepolarizing plate are used, for example.

Plastic films for image display devices typified by polarizer protectivefilms preferably have excellent mechanical strength. Therefore, orientedplastic films are preferably used as plastic films for image displaydevices.

Further, oriented plastic films preferably have excellent scratchresistance. Therefore, oriented plastic films with increased elasticmodulus have been proposed, as in PTLs 1 and 2.

CITATION LIST Patent Literature

PTL 1: JP 2019-8293 A (claim 4)

PTL 2: JP 2018-538572 T (claim 7)

SUMMARY OF INVENTION Technical Problem

As in PTLs 1 and 2, oriented plastic films with high elastic modulus canhave good scratch resistance when scratched with a hard material such asa pencil and a touch panel pen. However, oriented plastic films withhigh elastic modulus are more frequently scratched at an early stagewhen repeatedly rubbed with a soft material such as a cloth thanoriented plastic films with low elastic modulus.

Meanwhile, oriented plastic films with low elastic modulus havecomparatively good scratch resistance when repeatedly rubbed with a softmaterial such as a cloth, but some of them are immediately scratchedwhen scratched with a hard material such as a pencil and a touch panelpen.

As described above, there was a trade-off in having good scratchresistance of plastic films against both hard and soft materials.

It is an object of the present invention to provide an optical plasticfilm, a polarizing plate, and an image display device that can have goodscratch resistance against both hard and soft materials.

Solution to Problem

The present invention provides an optical plastic film, a polarizingplate, and an image display device as follows.

[1] An optical plastic film comprising: a first surface; and a secondsurface located on the opposite side to the first surface, the opticalplastic film satisfying the following condition 1:

<Condition 1>

under the premises that, regarding the indentation hardnesses of thecross sections in the conveyance direction of the plastic film, thelower value of the indentation hardness of the cross section on thefirst surface side and the indentation hardness of the cross section onthe second surface side is defined as MD1, and the indentation hardnessof the cross section in the middle in the thickness direction is definedas MD2, and regarding the indentation hardnesses of the cross sectionsin the transverse direction of the plastic film, the lower value of theindentation hardness of the cross section on the first surface side andthe indentation hardness of the cross section on the second surface sideis defined as TD1, and the indentation hardness of the cross section inthe middle in the thickness direction is defined as TD2, both MD2/MD1and TD2/TD1 are more than 1.01 and 1.30 or less.

[2] The optical plastic film according to [1] above, further satisfyingthe following condition 2:

<Condition 2>

when the larger of the product of MD1 and MD2 and the product of TD1 andTD2 is defined as X1, and the smaller is defined as X2, X1/X2 is 1.30 orless.

[3] The optical plastic film according to [1] or [2] above, furthersatisfying the following condition 3:

<Condition 3>

when a sample with a size of 50 mm in the conveyance direction×50 mm inthe transverse direction is cut out of the plastic film, slow axisdirections are measured at a total of five points, including four points10 mm advanced from the four corners of the sample toward the center andthe other point located at the center of the sample, and angles formedby either the conveyance direction or the transverse direction of thesample with the slow axis directions at the five points are definedrespectively as D1, D2, D3, D4, and D5, the difference between themaximum value of D1 to D5 and the minimum value of D1 to D5 is 5.0degrees or more.

[4] The optical plastic film according to any one of [1] to [3] above,further satisfying the following condition 4:

<Condition 4>

when a sample with a size of 50 mm in the conveyance direction×50 mm inthe transverse direction is cut out of the plastic film, in-planeretardations are measured at a total of five points including fourpoints 10 mm advanced from the four corners of the sample toward thecenter and the other point located at the center of the sample, and thein-plane retardations at the five points are respectively defined asRe1, Re2, Re3, Re4, and Re5, the average of Re1 to Re5 is 500 nm orless.

[5] The optical plastic film according to any one of [1] to [4] above,further satisfying the following condition 5:

<Condition 5>

when a sample with a size of 50 mm in the conveyance direction×50 mm inthe transverse direction is cut out of the plastic film, retardations inthe thickness direction are measured at a total of five points includingfour points 10 mm advanced from the four corners of the sample towardthe center and the other point located at the center of the sample, andthe retardations in the thickness direction at the five points arerespectively defined as Rth1, Rth2, Rth3, Rth4, and Rth5, the average ofRth1 to Rth5 is 2000 nm or more.

[6] A polarizing plate comprising: a polarizer; a transparent protectiveplate A disposed on one side of the polarizer; and a transparentprotective plate B disposed on the other side of the polarizer, whereinat least one selecting from the group consisting of the transparentprotective plate A and the transparent protective plate B is the opticalplastic film according to any one of claims [1] to [5] above.[7] An image display device comprising: a display element; and a plasticfilm disposed upon the light emitting surface side of the displayelement, wherein the plastic film is the optical plastic film accordingto any one of [1] to [5] above.[8] The image display device according to claim [7] above, furthercomprising a polarizer between the display element and the plastic film.

Advantageous Effects of Invention

The optical plastic film, the polarizing plate, and the image displaydevice of the present invention can have good scratch resistance againstboth hard and soft materials.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of a sample for measuring thecross-sectional hardness of a plastic film.

FIG. 2 is a cross-sectional view illustrating the measurement points forthe cross-sectional hardness on the first surface side and thecross-sectional hardness on the second surface side under conditions 1and 2.

FIG. 3 is a plan view illustrating the measurement positions at fivepoints under conditions 3 to 6.

FIG. 4 is a cross-sectional view illustrating an embodiment of the imagedisplay device of the present invention.

FIG. 5 is a cross-sectional view illustrating another embodiment of theimage display device of the present invention.

FIG. 6 is a diagram schematically illustrating the procedure of repeatedfolding test.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention will be described.

[Plastic Film]

The optical plastic film of the present invention includes a firstsurface and a second surface located on the opposite side to the firstsurface and satisfies the following condition 1:

<Condition 1>

under the premises that, regarding the indentation hardnesses of thecross sections in the conveyance direction of the plastic film, thelower value of the indentation hardness of the cross section on thefirst surface side and the indentation hardness of the cross section onthe second surface side is defined as MD1, and the indentation hardnessof the cross section in the middle in the thickness direction is definedas MD2, and regarding the indentation hardnesses of the cross sectionsin the transverse direction of the plastic film, the lower value of theindentation hardness of the cross section on the first surface side andthe indentation hardness of the cross section on the second surface sideis defined as TD1, and the indentation hardness of the cross section inthe middle in the thickness direction is defined as TD2, both MD2/MD1and TD2/TD1 are more than 1.01 and 1.30 or less.

<Measurement Under Conditions 1 and 2>

The conditions 1 and 2 define the indentation hardnesses of the crosssections in the conveyance direction of the plastic film and theindentation hardnesses of the cross sections in the transverse directionof the plastic film. In order to measure the cross-sectional hardness ofthe plastic film under the conditions 1 and 2, it is first necessary tofabricate a sample for measurement. The sample can be fabricated, forexample, by the following steps (A1) and (A2).

(A1) Two cut samples are fabricated by cutting an optical plastic filminto a size of 2 mm in the conveyance direction×10 mm in the transversedirection, and then two embedded samples are fabricated by embedding thecut samples S with a resin R as shown in FIG. 1. An epoxy resin ispreferable for the resin for embedding.

Each embedded sample can be obtained, for example, by disposing a cutsample in a silicone embedding plate (silicone capsulate), then pouringa resin for embedding, further curing the resin for embedding (in thecase of an epoxy resin, manufactured by Struers, mentioned below as anexample, the resin is preferably left standing for 12 hours at normaltemperature for curing), and taking out the cut sample and the resin forembedding wrapping the cut sample from the silicone embedding plate(silicone capsulate). Examples of the silicone embedding plate (siliconecapsulate) include those manufactured by DOSAKA EM CO., LTD. An epoxyresin for embedding that mixes, for example, “EpoFix”, a product name,and “Curing Agent for EpoFix”, a product name, manufactured by Struersat 10:1.2 can be used. The two cut samples are collected from adjacentregions (within a region of 50 mm×50 mm). Further, the two cut sampleseach have a size of 2 mm±0.2 mm in the conveyance direction×10 mm±1 mmin the transverse direction.

(A2) A sample with cross sections in the conveyance direction exposedfor measuring the indentation hardnesses of the cross sections in theconveyance direction is fabricated by cutting one embedded sample with adiamond knife perpendicularly along the conveyance direction. A samplewith cross sections in the transverse direction exposed for measuringthe indentation hardnesses of the cross sections in the transversedirection is fabricated by cutting the other embedded sample with adiamond knife perpendicularly along the transverse direction. Eachembedded sample is preferably cut passing through the center of the cutsample.

Examples of the apparatus for cutting the embedded sample include “UltraMicrotome EM UC7”, a product name, manufactured by Leica Microsystems.

Using the sample for measuring the indentation hardnesses of the crosssections in the conveyance direction fabricated as above, theindentation hardness of the cross section on the first surface side, theindentation hardness of the cross section on the second surface side,and the indentation hardness of the cross section in the middle in thethickness direction are measured, to calculate MD1 and MD2.

Likewise, using the sample for measuring the indentation hardnesses ofthe cross sections in the transverse direction fabricated as above, theindentation hardness of the cross section on the first surface side, theindentation hardness of the cross section on the second surface side,and the indentation hardness of the cross section in the middle in thethickness direction are measured, to calculate TD1 and TD2.

Herein, the indentation hardness of the cross section on the firstsurface side and the indentation hardness of the cross section on thesecond surface side in the conveyance direction and the transversedirection and the indentation hardness of the cross section in themiddle in the thickness direction each mean an average of fivemeasurements.

The indentation hardness of each cross section is measured by pushing aBerkovich indenter (material: diamond triangular pyramid)perpendicularly into the cut surface of the sample. As shown in FIG. 2,the indentation hardness of the cross section on the first surface sideand the indentation hardness of the cross section on the second surfaceside are measured at positions 2.0 μm inside from the first surface andthe second surface (positions (i) and (iii) in FIG. 2 correspond to themeasurement points). FIG. 2 corresponds to the xz cross-sectional viewof FIG. 1. Further, “d” in FIG. 2 means the thickness direction of thecut samples of the plastic film. Further, “(ii)” in FIG. 2 means themiddle position in the thickness direction of the cut samples of theplastic film.

The indentation hardness is preferably measured under the followingconditions.

<Measurement Conditions>

-   -   Indenter used: Berkovich indenter (model number: TI-0039,        manufactured by Hysitron Inc.)    -   Indentation condition: displacement control method    -   Maximum indentation depth: 200 nm    -   Load application time: 20 seconds (speed: 10 nm/sec)    -   Holding time: Held for 5 seconds at maximum indentation depth    -   Unloading time: 20 seconds (speed: 10 nm/sec)

The indentation hardness can be calculated as follows.

First, the indentation depth h (nm) corresponding to the indentationload F (N) is continuously measured, to plot a load-displacement curve.The load-displacement curve plotted is analyzed, and the indentationhardness H_(IT) can be calculated as a value obtained by dividing themaximum indentation load F_(max) (N) by the projected area A_(p)(mm²)where the indenter is in contact with the plastic film (the followingformula (1)).

H _(IT) =F _(max) /A _(p)  (1)

Here, A_(p) is a contact projected area obtained by correcting thecurvature of the indenter tip by the standard method for the apparatus.

Before measuring the indentation hardness, standardization is preferablycarried out.

Standardization can be performed, for example, by conducting anindentation test using standard samples with a known indentationhardness and a known composite elastic modulus and confirming that theindentation hardness and the composite elastic modulus obtained from thetest results fall within reference values.

Standardization is preferably carried out each time the sample ischanged. However, in the case of the same samples, it is preferable tocontinuously measure the indentation hardness a plurality of times inview of the work efficiency. That is, measurement is preferablyperformed as in (1) below. In (1) below, the order of the measurement ofthe indentation hardness of the cross section in the conveyancedirection and the measurement of the indentation hardness of the crosssection in the transverse direction may be changed. (1) Afterstandardization, samples for measuring the indentation hardnesses of thecross sections in the conveyance direction are subjected to measurementof the indentation hardness of the cross section on the first surfaceside, the indentation hardness of the cross section on the secondsurface side, and the indentation hardness of the cross section in themiddle in the thickness direction 5 times for each, to calculate MD1 andMD2. Again, the standardization is performed, and samples for measuringthe indentation hardnesses of the cross sections in the transversedirection are subjected to measurement of the indentation hardness ofthe cross section on the first surface side, the indentation hardness ofthe cross section on the second surface side, and the indentationhardness of the cross section in the middle in the thickness direction 5times for each, to calculate TD1 and TD2.

Further, in the case where the measurement of the indentation hardnesscontinues for a long time, standardization is preferably carried out atleast before the lapse of 12 hours. For example, even if standardizationis not performed each time the sample is changed, standardization ispreferably carried out at least before the lapse of 12 hours.

Herein, various measurements such as conditions 1 and 2, and conditions3 to 6, which will be described below, are performed in the atmosphereof a temperature of 23° C.±5° C. and a humidity of 40% to 65% RH, unlessotherwise specified. Further, samples are exposed to the atmosphere for30 minutes or more before the measurements.

<Conveyance Direction and Transverse Direction>

The optical plastic film has, for example, a sheet-like form and aroll-like form.

In the case of the roll form, the conveyance direction of the roll andthe transverse direction of the roll are easily identified.

Meanwhile, in the case of the sheet-like form, when the conveyancedirection and the transverse direction can be easily confirmed likeuniaxially oriented films, the conveyance direction and the transversedirection may be identified according to the confirmation (in the caseof a uniaxially oriented film, the slow axis direction is generally thetransverse direction).

In the case where it is difficult to confirm the conveyance directionand the transverse direction of the sheet, the conveyance direction andthe transverse direction may be identified as in the followingprocedures (1) and (2).

(1) In the case where the sheet is rectangular or square, the conveyancedirection or the transverse direction may be identified by the foursides constituting the rectangle or square. A sheet-like film isfabricated by punching a roll-like film. Then, in order to increase theyield of the sheet, it is necessary to punch the sheet along theconveyance direction and the transverse direction of the roll.Therefore, it is a technical common sense that, in the case where thesheet is rectangular or square, the directions of the four sidesconstituting the rectangle or square match the conveyance direction orthe transverse direction.(2) In the case where the sheet has a shape other than rectangle orsquare (oval, triangle, and polygon other than triangle), a rectangle ora square having the maximum area from which the shape does not protrudemay be drawn, and the conveyance direction or the transverse directionmay be identified as in (1) above based on the rectangular or squaredrawn.

In the identification by the aforementioned procedures (1) and (2), itis not possible to distinguish which of the two directions is theconveyance direction and which is the transverse direction. However,since the conditions 1 to 6 herein are parameters that are satisfiedeven when the conveyance direction and the transverse direction areidentified in reverse, as long as the two direction of the conveyancedirection and the transverse direction can be determined, the conveyancedirection and the transverse direction may be identified by theaforementioned procedures (1) and (2).

<Condition 1>

The condition 1 prescribes that MD2/MD1 and TD2/TD1 are both over 1.01and 1.30 or less.

The fact that MD2/MD1 and TD2/TD1 are both over 1.01 means that theindentation hardness of the cross section of the plastic film is largeron the inside than on the surface side. When the surfaces of twodifferent objects have almost the same hardness, the one having a harderinside can have better scratch resistance when the surfaces of theobjects are scratched with a hard material. Therefore, setting bothMD2/MD1 and TD2/TD1 to over 1.01 can improve the scratch resistanceagainst a hard material, regardless of the direction of scratching.

Meanwhile, the fact that MD2/MD1 and TD2/TD1 are both 1.30 or less meansthat the indentation hardness of the cross section of the plastic filmis not excessively larger on the inside than on the surface side. Whenthe surface of the plastic film is scratched with a soft material, asofter inside of the plastic film makes it easier to release the stressduring scratching. Therefore, setting both MD2/MD1 and TD2/TD1 to 1.30or less can improve the scratch resistance against a soft materialregardless of the direction of scratching.

In the condition 1, both MD2/MD1 and TD2/TD1 are preferably over 1.01and 1.20 or less, more preferably 1.02 or more and 1.15 or less, furtherpreferably 1.02 or more and 1.10 or less.

The absolute values of MD1, MD2, TD1, and TD2 are not specificallylimited, as long as they impart a suitable mechanical strength, and aregenerally 150 MPa to 350 MPa, preferably 170 MPa to 300 MPa, morepreferably 200 MPa to 270 MPa, further preferably 220 MPa to 250 MPa.

It is preferable that one embodiment of the optical plastic film of thepresent invention further satisfy the following condition 2:

<Condition 2>

when the larger of the product of MD1 and MD2 and the product of TD1 andTD2 is defined as X1, and the smaller is defined as X2, X1/X2 is 1.30 orless.

The fact that X1/X2 is small means that the anisotropy in hardnessbetween the conveyance direction and the transverse direction is small.Therefore, setting X1/X2 to 1.30 or less can suppress the occurrence ofscratches in a specific direction when a material (such as the tip of apen) hits the plastic film (hereinafter, this performance may bereferred to as “dent resistance”). Further, setting X1/X2 to 1.30 orless can make it easy to suppress rupture and creasing due to bendingthat remains after the bending test.

X1/X2 is more preferably 1.25 or less, further preferably 1.20 or less.The lower limit of X1/X2 is about 1.03, preferably 1.05 or more, morepreferably 1.10 or more.

It is preferable that one embodiment of the optical plastic film of thepresent invention further satisfy the following condition 3:

<Condition 3>

when a sample with a size of 50 mm in the conveyance direction×50 mm inthe transverse direction is cut out of the plastic film, slow axisdirections are measured at a total of five points, including four points10 mm advanced from the four corners of the sample toward the center andthe other point located at the center of the sample, and angles formedby either the conveyance direction or the transverse direction of thesample with the slow axis directions at the five points are definedrespectively as D1, D2, D3, D4, and D5, the difference between themaximum value of D1 to D5 and the minimum value of D1 to D5 is 5.0degrees or more.

The condition 3 prescribes that the difference between the maximum valueof D1 to D5 and the minimum value of D1 to D5 is 5.0 degrees or more.Setting the difference to 5.0 degrees or more can suppress blackout atleast within the region of the sample when visually recognized withpolarized sunglasses.

Conventional optical plastic films are designed so that the slow axisdirections are not shifted, whereas the optical plastic film satisfyingthe condition 3 is different from the conventional optical films in thatit is configured so that the slow axis directions are intentionallyshifted. Further, the plastic film satisfying the condition 3 ischaracterized in that it focuses on the unevenness of the slow axes in acomparatively small region of 50 mm in length×50 mm in width.

Further, satisfying the condition 3 can improve the bending resistanceof the plastic film, which is also preferred.

Meanwhile, in a general-purpose oriented film with the slow axesaligned, the film ruptures, or creasing due to bending strongly remainsafter the bending test. Specifically, a general-purpose uniaxiallyoriented film ruptures when the bending test is performed along the slowaxis, or creasing due to bending strongly remains when the bending testis performed in a direction orthogonal to the slow axis. Further, in ageneral-purpose biaxially oriented film, creasing due to bendingstrongly remains when the bending test is performed in a directionorthogonal to the slow axis.

The plastic film satisfying the condition 3 can suppress creasing due tobending that remains after the bending test and rupture, regardless ofthe folding direction, which is preferred.

The difference between the maximum value of D1 to D5 and the minimumvalue of D1 to D5 is more preferably 6.0 degrees or more, furtherpreferably 8.0 degrees or more, furthermore preferably 10.0 degrees ormore.

When the difference between the maximum value of D1 to D5 and theminimum value of D1 to D5 is excessively large, there is a tendency thatthe orientation of the plastic film decreases, and the mechanicalstrength decreases. Therefore, the difference is preferably 20.0 degreesor less, more preferably 17.0 degrees or less, further preferably 15.0degrees or less.

In the optical plastic film of one embodiment of the present invention,D1 to D5 are each preferably 5 degrees to 30 degrees or 60 to 85degrees, more preferably 7 degrees to 25 degrees or 65 degrees to 83degrees, further preferably 10 degrees to 23 degrees or 67 degrees to 80degrees.

Setting each of D1 to D5 to 5 degrees or more or 85 degrees or less canmake it easy to suppress blackout when visually recognized withpolarized sunglasses. Further, setting each of D1 to D5 to 30 degrees orless or 60 degrees or more can make it easy to prevent the decrease inmechanical strength due to the decrease in orientation of the plasticfilm.

It is preferable that one embodiment of the optical plastic film of thepresent invention further satisfy the following condition 4:

<Condition 4>

when a sample with a size of 50 mm in the conveyance direction×50 mm inthe transverse direction is cut out of the plastic film, in-planeretardations are measured at a total of five points including fourpoints 10 mm advanced from the four corners of the sample toward thecenter and the other point located at the center of the sample, and thein-plane retardations at the five points are respectively defined asRe1, Re2, Re3, Re4, and Re5, the average of Re1 to Re5 is 500 nm orless.

The condition 4 prescribes that the average of Re1 to Re5 is 600 nm orless. Setting the average of Re1 to Re5 to 600 nm or less makes it easyto prevent rainbow-pattern unevenness (rainbow unevenness) when visuallyrecognized with naked eyes at least within the region of the sample.

The average of Re1 to Re5 is more preferably 300 nm or less, furtherpreferably 250 nm or less, furthermore preferably 200 nm or less. Thelower limit of the average of Re1 to Re5 is not specifically limited butis generally about 50 nm, preferably 100 nm or more.

Re1 to Re5 are each preferably 600 nm or less, more preferably 300 nm orless, further preferably 250 nm or less, furthermore preferably 200 nmor less.

The difference between the maximum value of Re1 to Re5 and the minimumvalue of Re1 to Re5 is preferably 200 nm or less, more preferably 150 nmor less, further preferably 100 nm or less.

It is preferable that the optical plastic film of one embodiment of thepresent invention satisfy the following condition 5:

<Condition 5>

when a sample with a size of 50 mm in the conveyance direction×50 mm inthe transverse direction is cut out of the plastic film, retardations inthe thickness direction are measured at a total of five points includingfour points 10 mm advanced from the four corners of the sample towardthe center and the other point located at the center of the sample, andthe retardations in the thickness direction at the five points arerespectively defined as Rth1, Rth2, Rth3, Rth4, and Rth5, the average ofRth1 to Rth5 is 2000 nm or more.

Satisfying the condition 5 allow the degree of orientation of theoptical plastic film to approach even biaxial one and can impart goodmechanical strength to the optical plastic film. Further, satisfying thecondition 5 can make it easy to suppress blackout when visuallyrecognized from an oblique direction through polarized sunglasses.

The average of Rth1 to Rth5 is more preferably 3000 nm or more, furtherpreferably 4000 nm or more. The upper limit of the average of Rth1 toRth5 is about 10000 nm, preferably 8000 nm or less, more preferably 7000nm or less.

Further, Rth1 to Rth5 are each preferably 2000 nm to 10000 nm, morepreferably 3000 nm to 8000 nm, further preferably 4000 nm to 7000 nm.

The difference between the maximum value of Rth1 to Rth5 and the minimumvalue of Rth1 to Rth5 is preferably 200 nm or less, more preferably 150nm or less, further preferably 100 nm or less.

It is preferable that the optical plastic film of one embodiment of thepresent invention further satisfy the following condition 6:

<Condition 6>

the average of Re1/Rth1, Re2/Rth2, Re3/Rth3, Re4/Rth4, and Re5/Rth5 is0.10 or less.

As a ratio (Re/Rth) of the in-plane retardation (Re) to the retardationin the thickness direction (Rth) decreases, the degree of orientation ofthe optical plastic film approaches even biaxial one. Accordingly,setting the ratio to 0.10 or less can impart good mechanical strength tothe optical plastic film. The ratio is more preferably 0.07 or less,further preferably 0.05 or less. The lower limit of the ratio is about0.01.

Re1/Rth1, Re2/Rth2, Re3/Rth3, Re4/Rth4, and Re5/Rth5 are each preferably0.10 or less, more preferably 0.07 or less, further preferably 0.05 orless. The lower limit of each ratio is about 0.01.

<Measurement of Conditions 3 to 6>

The samples with a size of 50 mm in length×50 mm in width to be used inthe conditions 3 to 6 or the like are cut out of the plastic film at anypositions. The five measurement points in the conditions 3 to 6 are atotal of five points including one point at the center and four points10 mm advanced from the four corners of each sample toward the center(five points shown by black dots in FIG. 3).

The in-plane retardation (Re) in the condition 4 and the retardation inthe thickness direction (Rth) in the condition 5 are represented byformulas (1) and (2) below using nx as the refractive index in the slowaxis direction, which is a direction with the largest refractive indexat each measurement point, ny as the refractive index in the fast axisdirection, which is the direction orthogonal to the slow axis directionat each measurement point, nz as the refractive index in the thicknessdirection of the plastic film, and T [nm] as the thickness of theplastic film. Herein, in-plane retardation (Re) and retardation in thethickness direction (Rth) are values of them at a wavelength of 550 nm.

In-plane retardation (Re)=(nx−ny)×T[nm]  (1)

Retardation in thickness direction (Rth)=((nx+ny)/2−nz)×T[nm]  (2)

The slow axis direction, the in-plane retardation (Re), and theretardation in the thickness direction (Rth) can be measured, forexample, by using “RETS-100”, a product name, manufactured by OtsukaElectronics Co., Ltd., or “KOBRA-WR” or “PAM-UHR100”, product names,manufactured by Oji Scientific Instruments.

When the in-plane retardation (Re) and so on are measured by using“RETS-100”, a product name, manufactured by Otsuka Electronics Co.,Ltd., it is preferable to prepare measurement according to (A1) to (A4)below.

(A1) First, to stabilize the light source of RETS-100, the light sourceis turned on and then left to stand for 60 minutes or longer.Thereafter, a rotating-analyzer method is selected together with a θmode (a mode of retardation measurement in the angle direction and Rthcalculation). As a result of selecting the θ mode, the stage functionsas a tilted, rotating stage.(A2) Subsequently, the following measurement conditions are inputted toRETS-100.

(Measurement Conditions)

-   -   Retardation measurement range: rotating-analyzer method    -   Measurement spot diameter: φ5 mm    -   Tilt angle range: 0°    -   Measurement wavelength range: 400 nm to 800 nm    -   Average refractive index of plastic film (e.g., for PET film,        set N=1.617)    -   Thickness: thickness of separately measured by SEM or optical        microscope        (A3) Subsequently, background data are acquired with no sample        set in the apparatus. With applying a closed system to the        apparatus, this operation is carried out every time the light        source is turned on.        (A4) Thereafter, a sample is set on the stage in the apparatus,        and subjected to measurement.

In the condition 3, the direction serving as a reference (the conveyancedirection or the transverse direction) for the angle formed with theslow axis direction may be based on any of the conveyance direction andthe transverse direction, as long as the same direction is referred inall D1 to D5.

In the case where a plurality of samples with a size of 50 mm inlength×50 mm in width can be collected from a sheet-like plastic film, aratio of samples satisfying a predetermined condition among theplurality of samples is preferably 50% or more, more preferably 70% ormore, further preferably 90% or more, furthermore preferably 100% ormore.

Further, in the case where a plurality of samples with a size of 50 mmin length×50 mm in width can be collected from a roll-like plastic film,samples collected at a predetermined position in the transversedirection of the roll preferably satisfy a predetermined condition inmost of the conveyance direction of the roll. Satisfying such aconfiguration enables the plastic film to exert a predetermined effectby picking up the plastic film at a predetermined position in thetransverse direction of the roll.

<Plastic Film>

Examples of the layer configuration of the plastic film include amonolayer structure and a multilayer structure. Among them, a monolayerstructure is preferable.

As described later, it is preferable for good mechanical strength andsuppression of rainbow unevenness that the plastic film be an orientedplastic film having a small in-plane retardation. To provide an orientedplastic film with a small in-plane retardation, fine orientation controlsuch as making the orientation approach evenness in the verticaldirection and the horizontal direction is important. While the fineorientation control is difficult in multilayer structures because of thedifference of physical properties and so on among layers, the mentionedcontrol is easy in monolayer structures, which are preferred.

Examples of the resin component constituting the plastic film includepolyester, triacetylcellulose (TAC), cellulose diacetate, celluloseacetate butyrate, polyamide, polyimide, polyethersulfone, polysulfone,polypropylene, polymethylpentene, polyvinyl chloride, polyvinyl acetal,polyether ketone, polymethyl methacrylate, polycarbonate, polyurethane,and amorphous olefin (Cyclo-Olefin-Polymer: COP). Among them, polyesteris preferred in that good mechanical strength is easily obtained. Thatis, it is preferable that the optical plastic film be a polyester film.

Examples of the polyester constituting the polyester film includepolyethylene terephthalate (PET), polyethylene naphthalate (PEN), andpolybutylene terephthalate (PBT). Among them, PET is preferred in thatit is easy to control the in-plane retardation to a low value because ofthe low intrinsic birefringence.

The plastic film may contain an additive such as a UV-absorbing agent, alight stabilizer, an antioxidant, an antistatic agent, a flameretardant, a gelation inhibitor, and a surfactant.

The thickness of the plastic film is preferably 15 μm to 60 μm, morepreferably 20 μm to 55 μm, further preferably 30 μm to 50 μm. Settingthe thickness to 15 μm or more allows good mechanical strength withease. Further, setting the thickness to 60 μm or less can reduce thein-plane retardation with ease.

The haze of the optical plastic film as defined in JIS K7136: 2000 ispreferably 3.0% or less, more preferably 2.0% or less, and still morepreferably 1.0% or less.

The total light transmittance of the optical plastic film as defined inJIS K7361-1: 1997 is preferably 80% or more, more preferably 85% ormore, and still more preferably 90% or more.

To obtain good mechanical strength, the plastic film is preferably anoriented plastic film, and more preferably an oriented polyester film.Further, the oriented polyester film preferably has a monolayerstructure of a polyester resin layer.

The oriented plastic film can be obtained by orienting a resin layercontaining components constituting the plastic film. Examples oforienting methods include biaxial orienting such as successive biaxialorienting and simultaneous biaxial orienting and uniaxial orienting suchas longitudinal uniaxial orienting. Among them, biaxial orienting ispreferred because it is easy to obtain a low in-plane retardation and ahigh mechanical strength. Thus, it is preferable that the orientedplastic film be a biaxially oriented plastic film. Further, amongbiaxially oriented plastic films, biaxially oriented polyester films arepreferred, biaxially oriented polyethylene terephthalate films are morepreferred. In the biaxially oriented plastic films, it is preferablethat orientation ratios in the conveyance direction and the transversedirection approach each other, for making it easy to satisfy thecondition 2.

—Successive Biaxial Orienting—

In successive biaxial orienting, a casting film is oriented in theconveyance direction, and the film is then oriented in the transversedirection.

Orientation in the conveyance direction is typically achieved byrotational speed difference between oriented rolls, and may be carriedout in one step, or carried out in multiple steps by using a pluralityof oriented roll pairs. From the viewpoint of reducing unevenness ofoptical properties including in-plane retardation, it is preferable tobring a plurality of nip rolls close to the oriented rolls. Theorientation ratio in the conveyance direction is typically 2 to 15times, and, from the viewpoint of reducing excessive unevenness ofoptical properties including in-plane retardation, preferably 2 to 7times, more preferably 3 to 5 times, and still more preferably 3 to 4times.

From the viewpoint of reducing excessive unevenness of opticalproperties including in-plane retardation, it is preferable that theorientation temperature be from the glass transition temperature of theresin to the glass transition temperature+100° C. For PET, theorientation temperature is preferably 70 to 120° C., more preferably 80to 110° C., and still more preferably 95 to 110° C.

Regarding the orientation temperature, reducing the orientation sectionat a low temperature, for example, by rapidly raising the temperature ofthe film tends to decrease the average of the in-plane retardations.Meanwhile, increasing the orientation section at a low temperature, forexample, by slowly raising the temperature of the film tends to increasethe orientation and the average of the in-plane retardations, whiledecreasing the unevenness in the slow axes.

It is preferable to use a heater that generates turbulent flow duringheating for orientation. Heating with a wind containing turbulent flowcauses a temperature difference in a minute region in the film surfaceto cause a minute shift in the orientation axes due to the temperaturedifference, thereby making it easy to satisfy the condition 3.

Functions including slidability, adhesiveness, and antistatic propertiesmay be imparted to the film oriented in the conveyance direction throughin-line coating. As necessary, surface treatment such as coronatreatment, flame treatment, and plasma treatment may be applied beforein-line coating.

A coating film formed in such in-line coating has a thickness as smallas about 10 nm to 2000 nm (the coating film is further stretched throughorientation treatment). Herein, such a thin layer is not counted as alayer constituting the plastic film.

For orientation in the transverse direction, a tenter method is used,wherein a film is oriented in the transverse direction while beingconveyed with both ends of the film held with clips. The orientationratio in the transverse direction is typically 2 to 15 times, and, fromthe viewpoint of reducing excessive unevenness of optical propertiesincluding in-plane retardation, preferably 2 to 5 times, more preferably3 to 5 times, and still more preferably 3 to 4.5 times. It is preferablethat the transverse orientation ratio be higher than the longitudinalorientation ratio.

It is preferable that the orientation temperature be from the glasstransition temperature of the resin to the glass transitiontemperature+120° C., and it is preferable that the temperature increaseas going from the upstream to the downstream. Specifically, as thesection for lateral orientation is bisected, the difference between thetemperature in the upstream and the temperature in the downstream ispreferably 20° C. or more, more preferably 30° C. or more, still morepreferably 35° C. or more, and even still more preferably 40° C. ormore. For PET, the orientation temperature in the first step ispreferably 80 to 120° C., more preferably 90 to 110° C., and still morepreferably 95 to 105° C.

In order to impart flatness and dimensional stability, it is preferableto perform heat treatment for the plastic film subjected to successivebiaxial orienting as above in a tenter at the orientation temperature orhigher and lower than the melting point. Specifically, in the case ofPET, heat fixation is preferably performed in the range from 150 to 255°C., more preferably in the range from 200 to 250° C. At this time, heatfixation at a temperature as high as possible below the melting pointenables the crystallinity inside the film to be maintained and thecrystallinity on the film surface to slightly decrease, thereby makingit easy to satisfy the condition 1. From the viewpoint of reducingexcessive unevenness of optical properties including in-planeretardation, it is preferable to perform additional heat-treatingorientation at 1 to 10% in the former half of the heat treatment.

After being subjected to the heat treatment, the plastic film is slowlycooled to room temperature and then wound. As necessary, relaxationtreatment and so on may be used in combination in the heat treatment orslow cooling. The relaxation rate in the heat treatment is, from theviewpoint of reducing excessive unevenness of optical propertiesincluding in-plane retardation, preferably 0.5 to 5%, more preferably0.5 to 3%, still more preferably 0.8 to 2.5%, and even still morepreferably 1 to 2%. The relaxation rate in the slow cooling is, from theviewpoint of reducing excessive unevenness of optical propertiesincluding in-plane retardation, preferably 0.5 to 3%, more preferably0.5 to 2%, still more preferably 0.5 to 1.5%, and even still morepreferably 0.5 to 1.0%. The temperature in the slow cooling is, from theviewpoint of flatness, preferably 80 to 150° C., more preferably 90 to130° C., still more preferably 100 to 130° C., and even still morepreferably 100 to 120° C.

—Simultaneous Biaxial Orienting—

In simultaneous biaxial orienting, a casting film is introduced into asimultaneous biaxial tenter, conveyed with both ends of the film heldwith clips, and oriented simultaneously and/or stepwise in theconveyance direction and transverse direction. While there aresimultaneous biaxial orienting machines of pantograph type, screw type,drive motor type, and linear motor type, those of drive motor type orlinear motor type allow arbitrary change of the orientation ratio andrelaxation treatment at any place, and are thus preferred.

The ratio of simultaneous biaxial orienting is typically 6 to 50 timesin an area ratio, and, from the viewpoint of reducing excessiveunevenness of optical properties including in-plane retardation,preferably 8 to 30 times, more preferably 9 to 25 times, still morepreferably 9 to 20 times, and even still more preferably 10 to 15 times.

In simultaneous biaxial orienting, it is preferable for reduction ofin-plane orientation difference to set the orientation ratio in theconveyance direction and that in the transverse direction to beidentical to each other and set the orientation speeds in thosedirections to be almost identical to each other.

The orientation temperature in simultaneous biaxial orienting is, fromthe viewpoint of reducing excessive unevenness of optical propertiesincluding in-plane retardation, preferably from the glass transitiontemperature of the resin to the glass transition temperature+120° C. ForPET, the orientation temperature is preferably 80 to 160° C., morepreferably 90 to 150° C., and still more preferably 100 to 140° C.

In order to impart flatness and dimensional stability, it is preferableto perform heat treatment for the film subjected to simultaneous biaxialorienting subsequently in a heat fixation chamber in the tenter at theorientation temperature or higher and lower than the melting point. Theconditions for the heat treatment are the same as the conditions for theheat treatment after successive biaxial orienting.

<Flexibility>

It is preferable for the plastic film not to undergo the occurrence ofcracking or rupture after being subjected to 100000 cycles (morepreferably, after being subjected to 300000 cycles) of the folding testshown in Examples. It is preferable for the plastic film that when ameasurement sample thereof after being subjected to 100000 cycles (morepreferably, after being subjected to 300000 cycles) of the folding testshown in Examples is placed on a horizontal table, the angle of edgewarpage of the measurement sample from the table be 20 degrees orsmaller, more preferably 15 degrees or smaller. If the angle of edgewarpage of the sample is 15 degrees or smaller, this means beingresistant to creasing due to bending. Further, it is preferable for theplastic film to exhibit the above-described results (no occurrence ofcracking, rupture, or creasing due to bending, and the edge of thesample after the test raised at an angle of 20 degrees or less) in anyof the conveyance direction and the transverse direction of the plasticfilm.

<Thickness>

The optical plastic film is preferably 10 μm or more, more preferably 20μm or more, further preferably 25 μm or more, in view of the mechanicalstrength. Further, the optical plastic film is preferably 100 μm orless, more preferably 75 μm or less, further preferably 50 μm or less,for reducing the in-plane retardation. Setting the thickness to 50 μm orless is preferable also for improving the flexibility.

<Applications>

As described above, the plastic film of the present invention can havegood scratch resistance when scratched with a hard material such as apencil and a touch panel pen and when repeatedly rubbed with a softmaterial such as a cloth. Therefore, the optical plastic film of thepresent invention can be used suitably as a plastic film of an imagedisplay device, particularly, as a plastic film of an image displaydevice equipped with a touch panel.

Further, the plastic film of one embodiment of the present inventionsatisfying the condition 2 or 3 can suppress rupture and creasing due tobending that remains after the bending test regardless of the foldingdirection and therefore can be used suitably as a plastic film of acurved image display device or a foldable image display device.

Examples of the plastic film of such an image display device include aplastic film used as a base material for various functional films suchas a polarizer protective film, a surface protective film, anantireflection film, and a conductive film constituting a touch panel.

[Optical Laminate]

The optical plastic film of the present invention may further comprise afunctional layer such as a protective layer, an antireflection layer, ahard coating layer, an antiglare layer, a retardation layer, an adhesivelayer, a transparent conductive layer, an antistatic layer, and anantifouling layer, to form an optical laminate.

The functional layer of the optical laminate preferably include anantireflection layer. The antireflection layer is preferably disposed onthe outermost surface on the side with the functional layer of theplastic film.

Having an antireflection layer as the functional layer of the opticallaminate makes it easy to suppress rainbow unevenness.

Further, it is more preferable that the functional layer include a hardcoating layer and an antireflection layer. In the case where thefunctional layer includes a hard coating layer and an antireflectionlayer, the hard coating layer and the antireflection layer arepreferably disposed on the optical plastic film in this order.

A general-purpose hard coating layer and a general-purposeantireflection layer can be applied.

[Polarizing Plate]

The polarizing plate of the present invention includes: a polarizer; atransparent protective plate A disposed on one side of the polarizer;and a transparent protective plate B disposed on the other side of thepolarizer, wherein at least one selecting from the group consisting ofthe transparent protective plate A and the transparent protective plateB is the optical plastic film of the present invention described above.

The polarizing plate is used, for example, in order to impartantireflection properties by combination with a λ/4 retardation plate.In this case, the λ/4 retardation plate is disposed on the displayelement of the image display device, and the polarizing plate isdisposed on the viewer side of the λ/4 retardation plate. Further, inthe case where the polarizing plate is used for liquid crystal displaydevices, it is used for imparting the function of a liquid crystalshutter. In this case, the liquid crystal display device is disposed inthe order of a lower polarizing plate, the liquid crystal displayelement, and an upper polarizing plate so that the absorption axis ofthe polarizer of the lower polarizing plate is orthogonal to theabsorption axis of the polarizer of the upper polarizing plate. In thisconfiguration, it is preferable to use the polarizing plate of thepresent invention as the upper polarizing plate.

<Transparent Protective Plate>

The polarizing plate of the present invention uses the optical plasticfilm of the present invention described above as at least one selectingfrom the group consisting of the transparent protective plate A and thetransparent protective plate B. In a preferred embodiment, thetransparent protective plate A and the transparent protective plate Bare each the optical plastic film of the present invention describedabove.

In the case where one of the transparent protective plate A and thetransparent protective plate B is the optical plastic film of thepresent invention described above, the other transparent protectiveplate is not specifically limited but is preferably a transparentprotective plate with optical isotropy. The “optical isotropy” refers tohaving an in-plane retardation of 20 nm or less, preferably 10 nm orless, more preferably 5 nm or less. Examples of the transparent basematerial with optical isotropy include an acrylic film and triacetylcellulose (TAC) film.

Further, in the case where one of the transparent protective plate A andthe transparent protective plate B is the optical plastic film of thepresent invention described above, the optical plastic film of thepresent invention described above is preferably used as the transparentprotective plate on the light emitting side.

<Polarizer>

Examples of polarizers include sheet-type polarizers such as polyvinylalcohol films, polyvinyl formal films, polyvinyl acetal films, andethylene-vinyl acetate copolymer-based saponified films, which are dyedwith iodine, etc., and oriented; wire grid type polarizers composed ofmany metal wires arranged in parallel; coating-type polarizers to whicha lyotropic liquid crystal or a dichroic guest-host material is applied;and multilayer thin film type polarizers. Further, these polarizers maybe reflection type polarizers provided with the function of reflectingthe polarization component that is not transmitted.

The polarizer is preferably disposed so that the absorption axis thereofis substantially parallel or substantially perpendicular to theconveyance direction or the transverse direction of the optical plasticfilm. Being substantially parallel means being within 0 degrees±5degrees, preferably being within 0 degrees±3 degrees, more preferablybeing within 0 degrees±1 degree. Being substantially perpendicular meansbeing within 90 degrees±5 degrees, preferably being within 90 degrees±3degrees, more preferably being within 90 degrees±1 degree.

[Image Display Device]

The image display device of the present invention includes: a displayelement; and a plastic film disposed in the light emitting surface sideof the display element, wherein the plastic film is the optical plasticfilm of the present invention described above.

FIG. 4 and FIG. 5 are each a sectional view showing an embodiment of animage display device 100 of the present invention.

The image display device 100 in FIG. 4 and FIG. 5 includes an opticalplastic film 10 in the light emitting surface side (upper side in FIG. 4and FIG. 5) of a display element 20. In each of FIG. 4 and FIG. 5, theimage display device 100 includes a polarizer 31 between the displayelement 20 and the optical plastic film 10. In each of FIG. 4 and FIG.5, a transparent protective plate A (32) or a transparent protectiveplate B (33) is laminated on each surface of the polarizer 31. For theimage display device in FIG. 5, the optical plastic film 10 is used asthe transparent protective plate A (32).

The image display device 100 is not limited to the forms of FIG. 4 andFIG. 5. Although each member constituting the image display device 100is disposed at a certain interval in each of FIG. 4 and FIG. 5, forexample, the respective members may be integrated, for example, viaadhesive layers. The image display device may include a member not shown(another plastic film, functional layer, or the like).

<Display Element>

Examples of the display element include liquid crystal display elements,EL display elements (organic EL display elements, inorganic EL displayelements), and plasma display elements, and further examples are LEDdisplay elements such as micro-LED display elements.

If the display element of the display device is a liquid crystal displayelement, a back light is required in the surface of the liquid crystaldisplay element in the opposite side to the resin sheet.

The image display device may be an image display device provided with atouch-panel function.

Examples of the types of touch panels include resistance film type,capacitance type, electromagnetic induction type, infrared type, andultrasonic type.

A touch-panel function may be imparted within the display element asin-cell touch-panel liquid crystal display elements, and a touch panelmay be placed on the display element.

As described above, the optical plastic film of the present inventioncan suppress rupture and creasing due to bending that remains after thebending test. Therefore, in the case of being a curved image displaydevice or a foldable image display device, the image display device ofthe present invention can exert a more outstanding effect, which ispreferred.

In the case where the image display device is a curved image displaydevice or a foldable image display device, the display element ispreferably an organic EL display element.

<Plastic Film>

The image display device of the present invention includes the opticalplastic film of the present invention described above in the lightemitting surface side of the display element. Only one piece or twopieces or more of the plastic film may be used.

Examples of the plastic film disposed in the light emitting surface sideof the display element include a plastic film used as a base materialfor various functional films such as a polarizer protective film, asurface protective film, an antireflection film, and a conductive filmconstituting a touch panel.

<Other Plastic Films>

The image display device of the present invention may include otherplastic films without inhibiting the effects of the present invention.

The other plastic films are preferably those with optical isotropy.

EXAMPLES

Next, the present invention will be described in more detail withreference to Examples; however, the present invention is not limited inany way by these Examples.

1. Measurements, Evaluation

In the following measurements and evaluations, an atmosphere with atemperature of 23° C.±5° C. and a humidity of 40 to 65% RH was used.Before the measurements and evaluations, samples were exposed to theatmosphere for 30 minutes or longer.

1-1. In-Plane Retardation (Re), Retardation in Thickness Direction(Rth), and Slow Axis Direction

A sample of 50 mm in the conveyance direction×50 mm in the transversedirection was cut from the optical plastic film of each of Examples andComparative Examples fabricated or prepared in “2” described later. Thein-plane retardations, the retardations in the thickness direction, andthe slow axis directions were measured at a total of five pointsincluding four points 10 mm advanced from the four corners of the cutsample toward the center and the center of the sample. Table 1 shows theaverage of Re1 to Re5 or the like calculated from the measurementresults. The measuring device used was “RETS-100 (measurement spot: 5 mmin diameter), a product name, manufactured by Otsuka Electronics Co.,Ltd. The slow axis directions were measured in a range of 0 to 90degrees, taking the conveyance direction (MD direction) of the plasticfilm as a reference of 0 degrees.

1-2. Indentation Hardness of Cross Section

From the region of the sample of 50 mm in the conveyance direction×50 mmin the transverse direction cut in 1-1 above, two pieces of cut sampleswith a size of 2 mm in the conveyance direction×10 mm in the transversedirection were fabricated, and then the cut samples were embedded with aresin as in FIG. 1 to fabricate two embedded samples. The embeddedsamples were fabricated according to the suitable techniques shown in(A1) and (A2) in the text of the present specification.

One of the embedded samples was cut perpendicularly along the conveyancedirection with a diamond knife, to fabricate a sample A for measuringthe indentation hardness of the cross section in the conveyancedirection with the cross section in the conveyance direction exposed.The other embedded sample was cut perpendicularly along the transversedirection with a diamond knife, to fabricate a sample B for measuringthe indentation hardness of the cross section in the transversedirection with the cross section in the transverse direction exposed.The apparatus used for cutting the embedded samples was “Ultra MicrotomeEM UC7”, a product name, manufactured by Leica Microsystems. Further,each sample was cut passing through the center the cut sample in theembedded sample. When cutting, the sample was first roughly cut (roughtrimming) and finally trimmed under the conditions of “SPEED: 1.00 mm/s”and “FEED: 70 nm” with precision, so that the cut surface passingthrough the center of the sample was substantially flat. After precisiontrimming, it was confirmed with a microscope that there were noobstacles to the measurement such as foreign substances andirregularities on the cut surface.

Then, the indentation hardness of the cross section on the first surfaceside, the indentation hardness of the cross section on the secondsurface side, and the indentation hardness of the cross section in themiddle in the thickness direction were measured using the sample A, tocalculate MD1 and MD2. Likewise, the indentation hardness of the crosssection on the first surface side, the indentation hardness of the crosssection on the second surface side, and the indentation hardness of thecross section in the middle in the thickness direction were measuredusing sample B, to calculate TD1 and TD2. Table 1 shows the results. Asshown in the text of the present specification, MD1, MD2, TD1, and TD2each mean an average of five measurements.

The indentation hardness of the cross section on the first surface sideand the indentation hardness of the cross section on the second surfaceside were measured at positions 2.0 μm inside the first surface and thesecond surface, as shown in FIG. 2.

The indentation hardness was measured by perpendicularly pushing aBerkovich indenter (material: diamond triangular pyramid) against thecross section using “TI950 TriboIndenter”, a product number,manufactured by Hysitron Inc., as a measuring device and an applicationsoft (TriboScan Version 9.6.0.2) attached to the apparatus under thefollowing conditions.

In the above measurement, standardization in which an indentation testwas performed using a standard sample (fused quartz (5-0098)manufactured by Hysitron Inc.) with a known indentation hardness and aknown composite elastic modulus to confirm that the indentation hardnessand the composite elastic modulus obtained from the test results werewithin the reference values was conducted before measuring theindentation hardness of each sample. That is, after the standardization,the sample A was subjected to measurement of the indentation hardness ofthe cross section on the first surface side, the indentation hardness ofthe cross section on the second surface side, and the indentationhardness of the cross section in the middle in the thickness direction 5times for each. Then, after the completion of the measurement of thesample A, the standardization was performed again, and the sample B wassubjected to measurement of the indentation hardness of the crosssection on the first surface side, the indentation hardness of the crosssection on the second surface side, and the indentation hardness of thecross section in the middle in the thickness direction 5 times for each.

In Table 1, those satisfying the condition 1 (both MD2/MD1 and TD2/TD1of over 1.01 and 1.30 or less) are shown by “Y”, and those notsatisfying the condition 1 are shown by “N”.

<Measurement Conditions>

-   -   Indenter used: Berkovich indenter (model number: TI-0039,        manufactured by Hysitron Inc.)    -   Indentation condition: displacement control method    -   Maximum indentation depth: 200 nm    -   Load application time: 20 seconds (speed: 10 nm/sec)    -   Holding time: Held for 5 seconds at maximum indentation depth    -   Unloading time: 20 seconds (speed: 10 nm/sec)

1-3. Scratch Resistance 1 (Scratch Resistance Against Hard Material)

A test pencil having a hardness F specified by JIS S6006 is pressedagainst the surface of the optical plastic film of each of Examples andComparative Examples, to conduct a pencil hardness test (4.9 N load)specified by JIS K5600-5-4:1999. The test was conducted in both theconveyance direction and the transverse direction. As a result, thosewithout scratches on the surface of the plastic film in both directionswere evaluated as “A”, and those with scratches on the surface of theplastic film in at least one direction were evaluated as “C”.

1-4. Scratch Resistance 2 (Scratch Resistance Against Soft Material)

A cotton No. 300 flannel cloth was pressed against the surface of theoptical plastic film of each of Examples and Comparative Examples andrubbed thereagainst 1000 times back and forth under a load of 500 g/cm²,and then the presence or absence of scratches was visually inspectedunder the illumination of a fluorescent lump. The test was conducted inboth the conveyance direction and the transverse direction. Theapparatus used for the test was a Gakushin wear tester (product number“AB-301” manufactured by TESTER SANGYO CO., LTD.). As a result, thosewithout scratches on the surface of the plastic film in both directionswere evaluated as “A”, and those with scratches on the surface of theplastic film in at least one direction were evaluated as “C”.

1-5. Flexibility <Transverse Direction>

A strip-shaped sample of 30 mm in the short side (transversedirection)×100 mm in the long side (conveyance direction) was cut out ofthe optical plastic film of each of Examples and Comparative Examples.The sample was fixed to a durability tester (product name: “DLDMLH-FS”,manufactured by YUASA SYSTEM Co., Ltd.) at both ends (regions within 10mm from each tip were fixed) in the short side (30 mm), and a repeatedfolding test involving 180-degrees folding was carried out in 100000cycles. The folding frequency was 120 cycles per minute. More detailedprocedures of the folding test are as follows.

A strip-shaped sample after the folding test was placed on a horizontaltable, and the angle of edge warpage of the sample from the table wasmeasured. Table 1 shows the results. A sample that ruptured during thetest was evaluated as “ruptured”.

<Conveyance Direction>

A strip-shaped sample of 30 mm in the short side (conveyancedirection)×100 mm in the long side (transverse direction) was cut out ofthe optical plastic film of each of Examples and Comparative Examplesand evaluated as above.

<Details of Folding Test>

In the repeated folding test, as illustrated in FIG. 6(A), a side part10C of a plastic film 10 and a side part 10D facing the side part 10Cwere fixed with fixing parts 60 disposed in parallel. The fixing parts60 were slidable in the horizontal direction.

Next, as illustrated in FIG. 6(B), the fixing parts 60 were moved tobring close to each other, thereby deforming the plastic film 10 likefolding. Further, as illustrated in FIG. 6(C), the fixing parts 60 weremoved until the interval between the two side parts of the plastic film10 facing each other and fixed with the fixing parts 60 reached 2 mm,and thereafter the fixing parts 60 were moved in the reverse directionto relieve the deformation of the plastic film 10.

The plastic film 10 can be folded at 180 degrees by moving the fixingparts 60 as illustrated FIG. 6(A) to (C). The interval between the twoside parts of the optical film 10 facing each other could be set to 2 mmby carrying out the repeated folding test in such a manner that thebending part 10E of the plastic film 10 did not protrude out of thelowest level of the fixing parts 60 while the interval when the fixingparts 60 come closest was controlled to 2 mm.

1-6. Rainbow Unevenness

A sample (sample fabricated in 1-1) cut out of the optical plastic ofeach of Examples and Comparative Examples was disposed on the polarizingplate on the viewer side of the image display device with the followingconfiguration, so that the TD direction of the sample was parallel tothe horizontal direction of the screen. Then, the image display devicewas turned on in a dark room environment and observed with naked eyesfrom various angles to evaluate the presence or absence of rainbowunevenness based on the following criteria.

A: No rainbow unevenness visually recognized.B: Rainbow unevenness visually recognized in a part of the region.C: Rainbow unevenness visually recognized in most of the region.

<Configuration of Image Display Device>

(1) Backlight source: white LED or cold-cathode tube(2) Polarizing plate on light source side: TAC films were included asprotective films for both sides of a polarizer composed of PVA andiodine. It was disposed so that the direction of the absorption axis ofthe polarizer was perpendicular to the horizontal direction of thescreen.(3) Image display cells: liquid crystal cells(4) Polarizing plate on viewer side: A polarizing plate including a TACfilm as a polarizer protective film for a polarizer composed of PVA andiodine. It was disposed so that the direction of the absorption axis ofthe polarizer was perpendicular to the parallel direction of the screen.(5) Size: 10 inches diagonal

1-7. Blackout

A sample (sample fabricated in 1-1) cut out of the optical plastic ofeach of Examples and Comparative Examples was disposed on the polarizingplate on the viewer side of the image display device with theconfiguration shown in 1-6, so that the TD direction of the sample wasparallel to the horizontal direction of the screen. Then, while theimage display with the sample disposed was placed in the verticaldirection, the image display device fabricated in each of Examples andComparative Examples was visually recognized from the front throughpolarized sunglasses that absorb S polarization, to evaluate blackoutbased on the following criteria.

A: No blackout occurred throughout entire region.B: Blackout occurred in a part of the region.C: Blackout occurred in most of the region.

2. Fabrication and Preparation of Oriented Polyester Film Example 1

With a kneader, 1 kg of PET (melting point 258° C., absorption centerwavelength: 320 nm) and 0.1 kg of a UV-absorbing agent(2,2′-(1,4-phenylene) bis(4H-3,1-benzoxazinone-4-one) were melt-blendedat 280° C. to produce pellets containing the UV-absorbing agent. Thepellets and PET with a melting point of 258° C. were put into a uniaxialextruder, melt-kneaded at 280° C., extruded from a T-die, and cast on acasting drum the surface temperature of which was controlled to 25° C.to afford a casting film. The amount of the UV-absorbing agent in thecasting film was 1 part by mass with respect to 100 parts by mass ofPET.

The casting film obtained was heated with a group of rolls set to 95°C., then oriented in the conveyance direction at an orientation ratio of3.3 times while both sides of the film were heated with a radiationheater so that the film temperature at a point of 150 mm within a 400-mmorientation section (where the start point was an oriented roll A, theendpoint was an oriented roll B, and the oriented rolls A and B each hadtwo nip rolls) was 103° C., and thereafter temporarily cooled. Duringheating with the radiation heater, air at 92° C. was blown at 4 m/stoward the film from the opposite side to the film through the radiationheater, to generate turbulent flow on the front and back sides of thefilm and disturb the temperature uniformity of the film.

Subsequently, both surfaces of the uniaxially oriented film weresubjected to corona discharge treatment in air to set the wet tension ofthe base material film to 55 mN/m, and “a coating solution for slipperylayers containing a polyester resin with a glass transition temperatureof 18° C., a polyester resin with a glass transition temperature of 82°C., and silica particles with an average particle size of 100 nm” wasapplied to both surfaces of the film after being subjected to coronadischarge treatment by in-line coating, and thus slippery layers wereformed.

Next, the uniaxially oriented film was introduced into a tenter,pre-heated with hot air at 95° C., and then oriented in the transversedirection of the film at an orientation ratio of 4.5 times at atemperature of 105° C. in the first step and 140° C. in the second step.Here, two-step orientation was carried out in such a manner that as thesection for lateral orientation was bisected, the degree of orientationof the film (film width at point of measurement—film width beforeorientation) at the intermediate point of the section for lateralorientation reached 80% of the degree of orientation at the end of thesection for lateral orientation. The laterally oriented film wasdirectly subjected to heat treatment with hot air in the tenter atstepwise heat treatment temperatures from 180° C. to 245° C.,subsequently subjected to 1%-relaxation treatment in the transversedirection under the same temperature conditions, further rapidly cooledto 100° C., and then subjected to 1%-relaxation treatment in thetransverse direction. Thereafter, the film was wound to afford anoptical plastic film of Example 1 (biaxially oriented polyester film,thickness: 40 μm).

Example 2

An optical plastic film of Example 2 (biaxially oriented polyester film,thickness: 40 μm) was obtained in the same manner as in Example 1,except that the point at which the film temperature reached 103° C. waschanged to a point of 200 mm within the 400-mm orientation section.

Example 3

An optical plastic film of Example 3 (biaxially oriented polyester film,thickness: 50 μm) was obtained in the same manner as in Example 1,except that the thickness of the casting film in Example 1 wasincreased, the orientation ratio in the conveyance direction was changedfrom 3.3 times to 3.5 times, and the orientation ratio in the transversedirection was changed from 4.5 times to 5.0 times.

Example 4

An optical plastic film of Example 4 (biaxially oriented polyester film,thickness: 42 μm) was obtained in the same manner as in Example 1,except that the thickness of the casting film in Example 1 wasincreased, the orientation ratio in the conveyance direction was changedfrom 3.3 times to 3.5 times, and the orientation ratio in the transversedirection was changed from 4.5 times to 5.0 times.

Comparative Example 1

As an optical plastic film of Comparative Example 1, a commerciallyavailable biaxially oriented polyester film (product name: COSMOSHINEA4100, manufactured by TOYOBO CO., LTD., thickness: 50 μm) was prepared.

Comparative Example 2

As an optical plastic film of Comparative Example 2, the biaxiallyoriented polyester film with a three-layer structure (three layers ofcrystalline polyester/non-crystalline polyester/crystalline polyester)of Example 13 according to JP 2018-59078 A was fabricated.

TABLE 1 Example Comparative Example 1 2 3 4 1 2 Indentation MD1 (MPa)220.8 216.5 215.2 215.1 214.2 248 hardness MD2 (MPa) 227.9 238.1 244.7242.5 272.7 221 TD1 (MPa) 236.5 231.8 228.8 224.4 228.5 262.1 TD2 (MPa)244.8 257.2 263.5 260.8 298.3 251.2 MD2/MD1 1.03 1.10 1.14 1.13 1.270.89 TD2/TD1 1.04 1.11 1.15 1.16 1.31 0.96 Whether or not condition 1satisfied Y Y Y Y N N X1/X2 1.15 1.16 1.14 1.12 1.17 1.20 In-plane Re1186 218 1106 889 2181 150 retardation Re2 193 298 1217 910 2196 133 (nm)Re3 197 285 1195 927 2204 147 Re4 144 231 1131 905 2210 129 Re5 110 2461178 883 2218 144 Condition 4: average of Re 166 256 1165 903 2202 141Slow axis D1 76.93 78.44 72.44 77.67 58.37 79.30 direction D2 75.4084.54 74.16 76.28 58.06 79.07 (degree) D3 76.90 83.93 75.03 72.59 58.2479.14 D4 78.76 79.35 79.64 78.31 58.05 79.26 D5 68.34 81.40 79.15 71.2656.88 77.60 Condition 3: maximum 10.42 6.10 7.20 7.05 1.49 1.70 value −minimum value Retardation Rth1 6122 6115 14015 11022 8017 1255 inthickness Rth2 6139 6032 13957 10973 7955 1250 direction (nm) Rth3 61046040 13992 10961 7869 1248 Rth4 6129 6099 14074 11085 7925 1245 Rth56191 6064 14101 10929 8014 1525 Condition 5: average of Rth 6137 607014028 10994 7956 1305 Re/Rth Re1/Rth1 0.030 0.036 0.079 0.081 0.2720.120 Re2/Rth2 0.031 0.049 0.087 0.083 0.276 0.106 Re3/Rth3 0.032 0.0470.085 0.085 0.280 0.118 Re4/Rth4 0.023 0.038 0.080 0.082 0.279 0.104Re5/Rth5 0.018 0.041 0.084 0.081 0.277 0.094 Condition 6: average ofRe/Rth 0.027 0.042 0.083 0.082 0.277 0.108 Evaluation Scratch resistance1 A A A A A C Scratch resistance 2 A A A A C A Rainbow unevenness A A CC C A Blackout B B B B C C Bending resistance 10 degrees 10 degrees 15degrees 12 degrees  0 degrees 20 degrees (transverse direction) Bendingresistance 10 degrees 11 degrees 15 degrees 12 degrees 30 degrees 20degrees (conveyance direction)

From the results shown in Table 1, it can be confirmed that the opticalplastic films of Examples 1 to 4 satisfying the condition 1 can havegood scratch resistance against both of the hard material and the softmaterial. Further, it can be confirmed that the optical plastic films ofExamples 1 to 4 can suppress blackout by satisfying the condition 3 andfurther can suppress rupture and creasing due to bending that remainsafter the bending test, regardless of the folding direction. Further, itcan be confirmed that the optical plastic films of Examples 1 and 2 cansuppress rainbow unevenness by satisfying the condition 4.

REFERENCE SIGNS LIST

-   10: optical plastic film-   20: display element-   30: polarizing plate-   31: polarizer-   32: transparent protective plate A-   33: transparent protective plate B-   50: housing-   100: image display device-   S: cut sample of plastic film-   R: embedded resin-   d: thickness direction of cut sample of plastic film

1. An optical plastic film comprising: a first surface; and a secondsurface located on the opposite side to the first surface, the opticalplastic film satisfying the following condition 1: <Condition 1> underthe premises that, regarding the indentation hardnesses of the crosssections in the conveyance direction of the plastic film, the lowervalue of the indentation hardness of the cross section on the firstsurface side and the indentation hardness of the cross section on thesecond surface side is defined as MD1, and the indentation hardness ofthe cross section in the middle in the thickness direction is defined asMD2, and regarding the indentation hardnesses of the cross sections inthe transverse direction of the plastic film, the lower value of theindentation hardness of the cross section on the first surface side andthe indentation hardness of the cross section on the second surface sideis defined as TD1, and the indentation hardness of the cross section inthe middle in the thickness direction is defined as TD2, both MD2/MD1and TD2/TD1 are more than 1.01 and 1.30 or less.
 2. The optical plasticfilm according to claim 1, further satisfying the following condition 2:<Condition 2> when the larger of the product of MID and MD2 and theproduct of TD1 and TD2 is defined as X1, and the smaller is defined asX2, X1/X2 is 1.30 or less.
 3. The optical plastic film according toclaim 1, further satisfying the following condition 3: <Condition 3>when a sample with a size of 50 mm in the conveyance direction×50 mm inthe transverse direction is cut out of the plastic film, slow axisdirections are measured at a total of five points, including four points10 mm advanced from the four corners of the sample toward the center andthe other point located at the center of the sample, and angles formedby either the conveyance direction or the transverse direction of thesample with the slow axis directions at the five points are definedrespectively as D1, D2, D3, D4, and D5, the difference between themaximum value of D1 to D5 and the minimum value of D1 to D5 is 5.0degrees or more.
 4. The optical plastic film according to claim 1,further satisfying the following condition 4: <Condition 4> when asample with a size of 50 mm in the conveyance direction×50 mm in thetransverse direction is cut out of the plastic film, in-planeretardations are measured at a total of five points including fourpoints 10 mm advanced from the four corners of the sample toward thecenter and the other point located at the center of the sample, and thein-plane retardations at the five points are respectively defined asRe1, Re2, Re3, Re4, and Re5, the average of Re1 to Re5 is 500 nm orless.
 5. The optical plastic film according to claim 1, furthersatisfying the following condition 5: <Condition 5> when a sample with asize of 50 mm in the conveyance direction×50 mm in the transversedirection is cut out of the plastic film, retardations in the thicknessdirection are measured at a total of five points including four points10 mm advanced from the four corners of the sample toward the center andthe other point located at the center of the sample, and theretardations in the thickness direction at the five points arerespectively defined as Rth1, Rth2, Rth3, Rth4, and Rth5, the average ofRth1 to Rth5 is 2000 nm or more.
 6. A polarizing plate comprising: apolarizer; a transparent protective plate A disposed on one side of thepolarizer; and a transparent protective plate B disposed on the otherside of the polarizer, wherein at least one selecting from the groupconsisting of the transparent protective plate A and the transparentprotective plate B is the optical plastic film according to claim
 1. 7.An image display device comprising: a display element; and a plasticfilm disposed upon the light emitting surface side of the displayelement, wherein the plastic film is the optical plastic film accordingto claim
 1. 8. The image display device according to claim 7, furthercomprising a polarizer between the display element and the plastic film.